1. Field of the Invention
The invention relates to an electronic chopper circuit and to a method for operating it.
2. Description of the Prior Art
Such a circuit is known, for example, from the book by F. Zach, Leistungselektronic (Paraelectronics), Springer-Verlag, 1979, page 261. It is used for direct-current chopper converters or choppers for regulating the direct voltage in direct-current voltage changers with intermediate direct-voltage circuits. Essentially, it is composed of a main thyristor and an associated quenching circuit which is constructed as a controllable tuned circuit. As long as the main thyristor is cut off, the quenching capacitor is charged up via the charging diode from the voltage applied to the input. The regulating process begins with the firing of the main thyristor by means of a firing pulse applied to the control gate. The quenching thyristor is cut off at this time and thus prevents the quenching capacitor from being discharged or recharged. The input voltage drives a current through the load resistance via the main thyristor. The regulating process ends with the quenching thyristor firing as a result of a firing pulse being applied to its control gate. The charged quenching capacitor then swings over via the quenching thyristor and the quenching inductance and causes a so-called "soft" quenching in the main thyristor when it swings back.
With a predetermined input voltage, the mean voltage across the load resistance during periodic chopper operation is given by the ratio between on times and duration of the period, i.e., the control ratio. The control ratio is determined by the sequence in time of the firing pulses for the main and the quenching thyristor. The firing pulses, in turn, are generated in an external control logic in accordance with the desired mean output voltage. In order therefore to use the known circuit for limiting the input voltage, elaborate measuring and control electronics are required, which derive from the value of the input voltage applied a firing pulse sequence for the main and quenching thyristors which is suitable for limiting the voltage.